https://www.breitbart.com/faith/2021/03/06/pope-francis-iraqis-hate-greatest-blasphemy/

Matthew 12:31, "Wherefore I say unto you, All manner of sin and blasphemy shall be forgiven unto men: but the blasphemy against the Holy Ghost shall not be forgiven unto men." So blasphemy against the Holy Spirit would be the greatest blasphemy.